Piezo-electric device



Aprifi 28, 1931. c. B. SAWYER ETAL 1,302,781

. PIEZO-ELECTRIC DEVICE Filed May 6, 1927 Patented Apr. 28, 1931 of the electrostatic field varies.

UNITED- i STATES PA-TENT OFFICE CHARLES B. SAWYE'B AND CHARLES E, BRUSH, JR., OF CLEVELAND HEIGHTS, OHIO; THE CLEVELAND TRUST COMPANY, EXECUTOR OF SAID CHARLES I. BRUSH, JR DECEASED, ASSIGNORS, BY MES NE ASSIGNMENTS, TO THE CLEFELAND TRUST COM- PANY, OF CLEVELAND, OHIO, A CORPORATION 01 OHIO I rxnzo-nnnornro nnvIon This invention relates to piezo-electric'devices, and more particularly to animproved method of and means for utilizing the piezo eifect of piezo-electric material.

Piezo-electric crystals, such as, for example, Rochelle salt crystals, have been used in various devices, such as loud s eakers, transmitters and the like for trans orming sound pulsations into electric vibrations, and vice versa. In general, an entire crystal has been employed and a twisting motion or longitudinal expansion of the crystal obtained when an electric field has been applied thereto. The crystals so emplo ed have been either of the clear type or o the composite type. The former type comprises a clear crystalline structure, whereas the latter type is characterized by porous or composite portions at the ends, frequently known as hourglass regions. With either type of crystal the mechanical deflection obtained is small unless a relatively large crystal is employed, in which case the tone quality obtained is apt to suffer, since with such large crystals the natural period of the crystal, WhlCh 1s a function of the size, is frequently brought down into the audible range. Thuswhen the frequency of the applied excitation 1s substantially the same as the natural perlod, the

response is much greater for this frequency than for the other applied frequencies, whereby blasts or distortions result. 7

In employing complete crystals, furthermore, since such crystals are ordinarily of non-uniform cross-section, the length of path Such variation in the path of the electrostatic field necessarily results in a non-uniform potential gradient and therefore non-uniform piezo-electric effect in various portions of the crystal. Furthermore, the thinner sections of the crystal may therefore be near the breakdown point due to the applied voltage when the thicker sections of the crystal It will also be noted that piezo-electric crystals, for example, Rochelle salt crystals,

as previously used have been of non-uniform character, with attendant disadvantages. Moreover, when Rochelle salt crystals of the composite type have been employed such crystals have been subject to changes in characteristics, due to loss of, or increase of, moisture content in the porous end regions.

Furthermore, large and relatively perfect crystals are required when used for frequencies within the audiblerange, in some cases weighing one or two pounds apiece with the attendant difliculties of growing and expense of roduction.

n object of our invention is to provide a piezo-electric acoustic device of lmproved operating characteristics.

A further obj ect of this invention is to provide a piezo-elect-ric acoustic device of greater efliciency.

Another object of the invention is to provide piezo-electric acoustic devices of uniform quality and characteristics.

A further object of this invention is the more eflicient utilization of a piezo-crystalline material of the Rochelle salt type.

Another object of our invention is to provide piezo-electric crystalline material of improved character for use in acoustic devices.

A further object of the invention is to provide a piezo-electric acoustic device which will reproduce electrical impulses, faithfully longitudinal faces of substantially trapezoidal shape;

Fig. 8 is an elevational view showing an acoustic device according to this invention for cutting phonograph records; and

Fig. 9 is an elevational view illustrating a loud speaker constructed in accordance with our invention.

In our present invention we cut or otherwise obtain from a piezo-electric crystal or portion thereof, preferably though not i opposed or free end of said bar.

necessarily Rochelle salt, a bar which is preferably though not necessarily of materially greater length than the cross-sectional dimensions thereof and which may be of substantially rectangular cross-section. The longitudinal faces of the bar may also be of rectangular shape, although it is frequently desirable that the said faces be of trapezoidal or of other suitable shape.

In the case of bars of piezo-electric material of the Rochelle salt type, for example, double tartrates, such as of the alkali metals, obtained from such a piezo-electric crystal or portion thereof, a pair of opposed longitudinal faces thereof are preferably substantially parallel to the plane of the major longitudinal and major transverse crystalline axes, as hereinafter defined. Moreover, such a bar is preferably so cut that its greatestdimension is at an angle of substantially degrees to a major crystalline axis, although it will be understood that certain'of the advantages of this invention may be realized if said bars are cut at other angles. In general it may be stated that the bar is preferably so cut from a piezo-electrical material that when an electrostatic field is applied to said bar perpendicular to the said opposed longitudinal faces thereof, the bar will tendto expand or contract in the direction of its greatest longitudinal dimension or, in other words, of what may be termed its longitudinal axis.

We may provide an improved acoustic device according to this invention by suitably mounting a piezo-electric bar of the typeabove described. Thus, the bar may be secured at one end thereof to a support and an acoustic member such as a loud speaker cone or record-cutting jewel secured to the Suitable electrodes may then be disposed on opposed transverse axis bb and the minor axis a-a,

said axes being mutually perpendicular. There is indicated in the dotted lines in Figs. 1, 2 and 3 one manner in which a bar according to our present invention may be obtained from the crystal portion 1. Each bar 3 is preferably so cut from the crystal portion 1 that a pair pf opposed faces are substantially parallel to the bottom face 4 of the crystal portion .1 and to the major crystalline axes cc and bb. Where the bottom face 4 is relatively smooth, such face may form one of the faces of each bar to be cut therefrom. Moreover, the bars 3 are preferably so cut from a crystal portion 1 that the direction of major dimension of the bar or, as may be stated, the longitudinal axis of the bar, is at substantially 45 degrees to each of the major crystalline axes since bars so cut exhibit the greatest amount of longitudinal movement for a given electrostatic field. In other words, the longitudinal axis of the bar represents the direction of movement of the bar when subjected to an electrostatic field. It will be understood, of course, that one or more bars may be obtained from one crystal portion, depending upon the size of the crystal or crystal portion and the size of bar desired.

In Figs. 4; and 5 are shown a piezo-electric bar 3 of our invention having associated therewith suitable electrodes 5 to which may be connected suitable wires 6. The bar 3 is shown as being of materially greater length than the cross-sectional dimensions thereof, the bar being shown as having a substantially rectangular cross-section. The bar 3 is shown as having the ends 8 and the substantially parallel longitudinal faces 7. The dimensions of the bar 3 are dependent upon the particular conditions to which the bar is to be subjected. Frequently, however, it is found advantageous to have the distance between the opposed faces 7 relatively small since in such case the desired mechanical movement of the bar due to the piezo-electrical effect may be obtained from relatively small applied voltages since it is well understood that the mechanical movement of the crystal is dependent upon the volts per inch, or in other words, the potential gradient through the crystal. Conversely, mechanical movement crystalline axes,

pendent upon the bar is to be put.

of relatively magnitude will produce voltages of a preciable ma 'tude.

When the bar shown in%i gs. 4 and 5'1ssubjected to an electrostatic field. erpen- In Fig. 6 I have shown a microphone embodying my invention. Thus, to the casing- 9 is secured a relatively heavy sup ort 10 which may be of lead or other suita 1e material which will remain stationary and not be affected b vibrations of relatively-high frequency. he support 10 is shown as be- 1 ing substantiall U-shaped. The piezo-electric bar 18 may secured at one end thereof to the support 10 by suitable dhesive material 11, such as shellac, Canada balsam or the like. To the outer or actuating end of the bar 18 may be secured a socket member 12 which in turn carries a suitable'acoustic diaphragm, such as the cone 13. Shellac or other suitable adhesive material 14 may also be employed to secure the socket member 12 to the outer or actuatin end of the bar 18. Electrodes. 19, such as of tin foil, may be disposed adjacent or secured to the opposed longitudinal faces 20 of the bar 18. Suitable wires 15 ma be connected to the electrodes 19 and to binding posts 16 insulated as by the insulative washers 17 from the casing 9.

When sound wavesimpinge upon the cone 13, the bar 18 is stressed along its longitudinal axis whereby, due to the piezo-electric effect, an electromotive force is generated between the electrodes 19. By connecting suitable apparatus to the binder posts 16, therefore, such varying voltage may be transmitted to suitable apparatus such as broadcasting apparatus, sound reproducing apparatus or the ing electrostatic field is applied to-the bar 18.

Due to the iezo-electric effect and the position of the ongitudinal axis of the bar with respect to the crystalline axes, such varying electrostatic field causes the bar 18 to contract or expand along its longitudinal axis, thus actuating the cone to reproduce sounds corresponding to the electric vibrations transmitted to the bar.

, by increasing .The longitudinal faces 20 of the bar shown in Fig. 6 maybe of rectangular shape as shown in Fig. 4 or of substantially trapezoidal shape as indicated by the faces 21 of the bar 22"shown in Fig. 7,01 of any suitable shape. The natural period of a bar W n expanding and contracting longitudina y is proportional to itslength. A bar of the shape shown in Fig. 7 therefore may be considered as made up of an infinite number of bars of unequal length thereforetending to respond to an infinite number of natural er1ods."Experience has shown that when a ar of thissha e is used'for a microphone or loud -s a er, very desirable acoustic lcharacteristics are obtained.

Referring again to the structure shown in Fig. 6, the equivalent electrostatic capacity of the microphone may, of course, be increased as by decreasing the distance between the electrodes by makin the bar thinner, or by increasing the area 0 the electrodes the area of the longitudinal faces of the ar, as is the case in any dielectric material? or vice versa, so that the impedance may be given the value most suitable for any thermionic or other electrical device with which the microphone may be used. Ifthe longitudinal dimensions of the bar are reasonably small the natural period or periods, of'the bar 18 will be well above the audible range and disagreeable resonances or blasts prevented.

In carrying out our invention, instead of securing a piezo-electric bar at one end thereof to a relatively heavy support and at the other end thereof to an acoustic member, as in Fig. 6, we may secure acoustic members to each end of said bar. Thus, acoustic diaphragms, such as cones, may be securedto the ends of a bar to form, for example, a

loud speaker. In such case the bar may be supported or suspended as by-resilient material cooperating with the' central portion thereof.

In Figs. 8 and 9 we have shown devices embodying our invention in which the longitudinal movement ofa bar under the influence of an electrostaticfield is mechanically magnified. Thus in Fig. 8 is shown a device for cutting phonograph records. To the arm 23 is secured a support 24 preferably of lead or other suitable material. At the outer end of the arm 23 is secured a lever 25 which may be of channel formation as shown at 26 to provide stiffness. The lever 23 may thus pivot at 27 adjacent the arm 23. A piezoelectric bar 28 according to our invention is shown disposed intermediate the support 24 and the lever 25,- one end of the said bar being secured to said support by adhesive material such as shellac 29, the outer or actuating end of the bar being secured-by suitable adhesive material 29 such as shellac to a socket member 30 which is in turn'dispos'ed adjacentand secured to the lever 25. Electrodes 31 of tin foil or the like may be disposed ad'acent or secured to opposed longitudinal aces of the bar 28 to which electrodes may be secured the wires 32. To the lower end of the lever is shown secured a cutting jewel or member 33 arranged to cut a suitable groove in a record blank 34.

Whenthe bar 28 is subjected to a varying voltage by means of the wires 32 a varying electrostatic field will be set up in the bar 28 causing the bar 28 to contract and expand longitudinally or in the direction of its greatest dimension. Such longitudinal movement will be transmitted to the lever 25 giving the cutting jewel 33 a magnified motion in the directions indicated by the arrows in Fig. 8. In this manner a suitable spiral groove may be cut in the record blank 34. It will be understood that during the process of cutting the record the blank 34 will be rotated and simultaneously moved bodily in a direction toward the right, referring to Fig. 8, whereby a spiral groove may be cut in the blank. I,

In Fig. 9 we have illustrated a loud speaker device embodying our invention. The relatively heavy frame portion 35 having the upstanding portions 36 and 37 may be composed of lead or other suitable material. To the frame 35 is firmly secured a fixed fulcrum member 38, cooperable with the lever 39. A yieldable fulcrum member 40 cooperates with the opposed side ofthe lever and is supported by a relatively heavy or stiff spring 41, the tension of which may be regulated by the screw 42 carried by the frame portion 37. A cone 43 mounted on a shaft 44 is secured to the lever 39. Disposed between the frame member' 36 and the lever 39 is a piezo-electric bar 45 to each of the ends of which are secured, by suitable cement, the end pieces 46 and 47 arranged to cooperate with the frame portion 36 and lever 39 respectively, which end pieces 46 and 47 may be omitted if desired. On opposed longitudinalfaces .of the bar are disposed electrodes 48 such as of tin foil, to which electrodes are secured the wires 49. When the bar 45 is subjected to an electrostatic field by means of the wires 49 and electrodes 48, the bar 45 will expand longitudinally and move the lever 39 and cone 43 forwardly, toward the right in Fig. 9. If the electrostatic field is reversed the bar will contract, and the lever 39 and cone 43 will be moved rearwardly by the fulcrum member 40. Due to the leverage means, shown the cone 43 is given a magnified vibratory motion corresponding to the electrical impulses applied between the electrodes 48.

It will thus be seen that we have provided a piezo-electric bar and acoustic devices embodying the same of advantageous operating characteristics.

It will furthermore be noted that we have provided piezo-electric bars of economical characteristics in that a plurality of bars may be obtained from a single piezo-electric crystal or portionthereof. Our improved bars may moreover be obtained from imperfect crystals or portions thereof.

It willfurthermore be seen that we have provided piezo-electric bars which may be given uniform and constant piezo-electric characteristics since porous and imperfect crystalline structures may be avoided in cutting said bars.

It will also be noted that by means of our invention we may provide piezo-electric bars and devices embodying the same of uniform dimensions and operating characteristics.

It will further be seen that we have provided piezo-electric bars and devices embodying the same whose operating characteristics may be varied to suit the voltages available or to bring'the'impedance of said bars closer to a desired impedance, as when said devices are used as loud speakers or record-cutting devices.

It will moreover be noted that we have provided piezo-electric bars whose dimensions may be varied to bring the natural period thereof to any desired value.

It will also be seen that by our invention we may provide a piezo-electric bar responding to an infinite number of natural periods whereby when such bar is embodied in an acoustic device good reproduction throughout the musical range is efiected and undesirable resonances and blasts prevented.

It will moreover be understood that our invention is not limited to bars formed from Rochelle salts but that any suitable piezoelectric material may be utilized in the construction thereof.

To those skilled in the art many modifications of and widely differing embodiments and, applications of our invention will suggest themselves without departing from the spirit and scope thereof. Our disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.

What we claim is:

1. In an acoustic device of the class described, in combination, a support not appreciably affected by electrical or mechanical vibrations of audible frequency, a bar of piezo-electric material of the Rochelle salt type secured at one end thereof to said support,said bar being of substantially rectangular cross-section and of materially greater length than the cross-sectional dimensions thereof, said bar having a pair of opposed 1 longitudinal faces parallel to the plane of electric material of the Rochelle salt type cemented at one end to said support, said bar being of materially greater length than the cross-sectional dimension and having its longitudinal axis disposed at substantially 45 degrees to one of the major crystalline axes, electrodes disposed on opposite faces of the bar, means for producing an electrostatic field, and an acoustic member secured directly to the unsupported end of said bar.

3. In an acoustic device for receiving or radiating sound waves, in combination, a support of suflicient mass as to be substantially unafi'ected by vibrations of audible frequency, a bar of piezo-electric material of the R0- chelle salt type cemented at one end to said support, said bar being of materially greater length than the cross-sectional dimension and having its longitudinal axis disposed at substantially 45 degrees to one of the major crystalline axes, electrodes disposed on opposite faces of the bar, means for producing an electrostatic field, and an acoustic diaphragm for receiving or radiating sound waves secured directly to the unsup rted end of said bar.

4. In an acoustic evice for receiving or radiating sound waves, in combination, a support of sufficient mass as to be substantially unalfected by vibrations of audible frequency, a bar of piezo-electric material cemented at one end to said support, said bar being of materially greater length than the cross-sectional dimension and having its longitudinal axis disposed at substantially 45 de ees to one of the major crystalline axes, t e cross section of said bar decreasin from the suported end to the unsupporte end electrodes disposed on opposite faces of the bar, means for producin an electrostatic field, and an acoustic diap ragm for receiving or radiating sound waves secured to the unsupported end of said bar.

5. In a sound reproducing device in combination, a su port of suflicient mass as to be substantial y unaffected b vibrations of audible frequency, a bar 0 piezo-electric material of the Rochelle salt type rigidly secured at one end to said support, said bar being of materially greater length than the cross-sectional dlmension and having its longitudinal axis dis sed at substantially 45 degrees to one of t e major crystalline axes of the piezo-electric material, electrodes (11sosed on opposite faces of the bar substantialy parallel to the plane determined by the major crystalline axes, a sound radiating diaphragm rigidly secured directly to the unsupported end of the bar, whereby upon the production of a variable electrostatic field between the electrodes said bar and the sound radiating diaphragm will be caused to vibrate.

Intestimony whereof we affix our signatures.

CHARLES B. SAWYER. CHARLES F. BRUSH, JR. 

